1. Field of the Invention
The invention pertains to a process and to a device for the automatic rectification of single-channel or multi-channel images, where an image is rectified onto a reference image without precise knowledge of the mapping function.
2. Description of the Related Art
Imprecise and/or insufficient knowledge of the mapping function is present especially when images are recorded by moving recording systems and/or moving image fields and the orientation of the recording system with respect to the image field cannot be determined clearly or can be determined only with extraordinary effort. For example, the mapping functions for rectifying satellite images for land surveying are not known or are known only within certain limits. It is also conceivable that the distortion of the reference image is not known or is known only within certain limits.
The images are recorded by a recording device and are then available for further processing as single-channel images, such as gray-scale images, or as multi-channel images, such as color images.
Images for recording and mapping the surface of the Earth are recorded by moving recording systems. The recording system can be located on, for example, a satellite or an aircraft. Known methods for recording and mapping the surface of the Earth are so-called photogrammetric processes, where the recorded images are rectified before they are transferred to the map to be prepared. The necessary transformation of a recorded image into the desired form of appearance is called the mapping process. So that rectification can be performed, the mapping process is modeled as a collinear equation. There are already several basic methods for georeferencing remote-acquired data, which are based either on the principle of calculating positions by means of a model of the mapping process (mapping model) or on the basis of the “control point principle” by means of a reference image. It is also possible to combine the two methods.
For rectification with the use of a mapping model, the mapping process is described as a collinear equation or is approximated by means of simpler models. If all the parameters of the mapping process are known, it is possible to generate very precise images. In practice, however, not all parameters which play a role in the formation of the image, such as the location and orientation of the satellite, are known with sufficient accuracy. The reasons for this are that there is a limit on the measurement accuracy and the fact that not all parameters are stable over time. This instability over time is the result, for example, of variations in the orbit of the satellite. In addition, the orbit of the satellite is measured discretely, and the interpolation between these points in time results in additional error.
It is also known that, in the case of incomplete or imprecise mapping parameters, rectification can be carried out with the help of control points. This process is used especially when the image to be rectified, a satellite image, for example, is mapped not onto a defined geometry such as map but rather onto a reference image with unknown distortion. Control points are high-contrast image structures, which are stable over time and which make it possible to determine locations by means of correlations. The interpolation between the control points is usually accomplished with two-dimensional polynomial functions. A source of error in control point rectification is the dissimilarity of the control point structures in the different images, caused by different recording geometries, by different conditions of the objects that form the control point structure, or by the averaging of different objects or parts of objects. Another source of error is the polynomial function used to perform the interpolation, since this function usually does make it possible to model the mapping process with sufficient accuracy. This is especially true in the case of large areas. The greater distance of a point from a control point, the greater the effects attributable to inaccuracies in the polynomial function. This effect is evident especially in the case of higher-order polynomials.
It is also known that the two methods, i.e., parametric rectification and rectification by means of polynomial functions, can be combined.
A disadvantage of the known methods is that they are difficult to automate, especially when the knowledge of the mapping process is very limited. It is usually a very complicated matter to adjust all the mapping parameters to an actual situation; this difficulty often leads to the use of pure polynomial rectification. The necessary control points are simply taken from a database. The previously known methods thus have residual errors in the position determination of the control points of the data set to be processed. This residual error cannot be described analytically at all or only with great effort, which means that it is impossible to adjust the parameters appropriately by correcting the mapping model.